Polyhydroxyalkanoates (PHAs) are homopolymers or copolymers of hydroxyalkanoates, such as 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), 4-hydroxyvalerate (4HV) and 3-hydroxyhexonate (3HH). These thermoplastic or elastic biopolyesters are synthesized and accumulated by many microorganisms, bacteria in particular, as carbon and energy storage materials. PHAs are conveniently synthesized by cultivating the microbial cells in an aqueous medium on a carbon source, including sugars, organic acids and alcohols. Depending on the species, carbon source, nutrients and culture conditions, the PHA biopolymers may account for up to 80 wt % of dry cell mass. Their average molecular size ranges from 1,000 to 2,000 kDa. PHAs can also be formed and accumulated in transgenic plant cells.
The PHA biopolymers are stored inside of the cells as discrete granules of about 0.2-0.6 μm in diameter. The inclusions contain about 5 to 10 wt % of water, and are largely amorphous. Each granule is surrounded by a phospholipid monolayer membrane in which proteins, including the PHA synthase and degradase, are located. Other proteins (phasins) are presumed to be involved in stabilization of the amorphous hydrophobic PHA inclusions suspended in cell cytoplasm. Although the dried “plastic” cell mass with a high PHA content (70-80 wt %) can be directly molded into articles, only purified PHA polymers can have the desired thermal and mechanical properties for a variety of applications in packaging, agriculture and health care. Compared to other bioproducts of small and large molecules, PHA recovery poses a unique challenge, due to the solid state of PHA granules and non-PHA cell mass (NPCM). Two strategies are typically used in the recovery and purification of PHA from cell mass: PHA solubilization or NPCM dissolution. In the former, PHA macromolecules are dissolved in an appropriate organic solvent and extracted from the cells (NPCM), while in the latter, NPCM is digested and/or dissolved by agents while PHA polyesters are left in solid state form. The generated solid and liquid phases are separated by a unit operation such as filtration and centrifugation.
Solvent extraction of PHA is widely used in laboratories to prepare small quantities of high molecular weight PHA. It has also been used for PHA recovery at pilot-scale and large-scale, to a limited success. Only a few solvents are able to dissolve PHA macromolecules, particularly those of large molecular weight with a high content of short chain hydroxyalkanoates, such as 3-hydroxybutyrate and 3-hydroxyvalerate. Popular solvents are halogenated hydrocarbons such as chloroform and dichloromethane. The wet PHA-containing cells can be directly extracted with the water immiscible solvents, but pretreatment of cell mass is usually needed and performed, such as water removal at elevated temperature and extraction of lipids/pigments by PHA insoluble solvents, such as acetone and methanol. The pretreated cell mass is further subjected to extraction in hot chloroform or other appropriate solvents, and the dissolved PHA is separated from non-soluble cell mass by filtration and/or centrifugation. Unfortunately, a very viscous PHA solution is usually formed even at a relatively low PHA concentration (5% w/v), which renders such separation difficult. Precipitation of PHA by adding a PHA insoluble solvent (e.g., methanol) into the filtrate completes the separation and purification of PHA biopolymers from the solution. In general, solvent extraction results in pure and intact PHA macromolecules, but its major drawbacks include: (a) a large amount of organic solvent needed to make a dilute solution (less than 5 wt % PHA), (b) lengthy separation of the PHA solution from cell mass because of high viscosity of the solution, (c) high capital and operation costs for solvent recovery, and (d) loss of a large amount of volatile and possibly mutagenic organic solvents into the environment.
The PHA biopolymers can also be recovered and purified by digesting and/or solubilizing the non-PHA cell mass (NPCM), which leaves PHA granules in solid state. The NPCM comprises peptidoglycan, proteins, nucleic acids, phospholipids, and lipopolysaccharides. The cellular debris in aqueous solution, after digestion, can be easily removed from PHA granules by centrifugation and washing. Depending on the digestion agents and process conditions, the solubilization of non-PHA cell mass can be further classified into non-selective and selective dissolution. Sodium hypochlorite and sodium hydroxide are the representative agents for the former, while enzymes and anionic surfactants are representative of the latter.
Hypochlorite is a non-selective, powerful oxidation agent that digests both non-PHA biomass and PHA granules, resulting in a low recovery yield of PHA and reduced molecular weight (22 kDa) of purified polymers with poor mechanical strength. Many factors affect the purity, recovery yield and molecular size of PHA polymers, including temperature, pH, time and pretreatment of cell mass. Particularly, the dose of hypochlorite must be carefully adjusted according to the concentration of non-PHA cell mass, which is more often than not a variable, changing batch to batch in industrial fermentations. Careful control on hypochlorite ranging from 11 to 18 parts per part of NPCM is recommended to achieve a good purity, high molecular weight of PHA, but a moderate PHA recovery yield (˜75%).
A high purity PHA (98%) may also be obtained by using a strong alkaline solution (pH 13.6), but at the expense of PHA recovery yield and molecular size. The native amorphous PHA granules are actually quite vulnerable to alkaline saponification and quickly decomposed into soluble products such as monomers and oligomers. Similar to hypochlorite, the base is consumed during solubilization of PHA and NPCM. Therefore, an appropriate concentration of the agents or digestion time must be controlled according to the residual non-PHA cell mass. Furthermore, the alkaline non-selective dissolution is limited to treatment of a cell mass of very high PHA content (70 wt % or above).
Although the purity, yield and molecular size of PHA recovered by non-selective dissolution have to be compromised, they can be significantly improved by using a selective dissolution in which the non-PHA cell mass is solubilized and PHA granules are left intact. Proteolytic enzymes, for example, have high activities on hydrolysis and dissolution of proteins, but little activity on PHA degradation. Typical processing of a PHA-containing cell slurry (60 wt % PHA) starts with heat treatment, followed by enzymatic hydrolysis, surfactant treatment and finally hydrogen peroxide decolorization.
Anionic surfactants such as sodium dodecyl sulfate (SDS) can also help dissolve the non-PHA cellular mass to some extent, with little degradation of polyesters. The treatment, however, uses a high dosage of surfactant (0.24 g surfactant per g cell mass), which would not only raise the recovery cost, but also cause problems in wastewater treatment. Although up to 50% of solvent extraction cost can be saved by using a sequential surfactant and hypochlorite digestion, the moderate polymer recovery yield (˜75 wt %) and relatively high cost of chemical agents such as SDS and hypochlorite are the unsolved problems in production of biodegradable plastics, which can compete with synthetic polymers for a variety of environmentally friendly applications.